Bosch freezer rapid temperature rise on opening door, very slow to refreeze?

Hi

I have an upright Bosch freezer - product codes are: GSU2320GB/52, FD8012, 00804.

The freezer maintains a temperature of 18 degrees C but if the door is opened even for a short period i.e. the time it takes to take out an item, the temperature drops by up to 4 degrees - it then takes 6 or 7 hours to recover back to 18 degrees even with the use of the "super" button. I have defrosted the freezer as per the manual instructions several times now and this has not solved the problem. Why should there be such a rapid drop in temperature when the door is opened? It can't be anything to do with the seals since the freezer maintains its temperature when the door is closed. The other problem is why it takes so long for the freezer to reach 18 degrees again. Are you able to advise on whether the freezer can be repaired or has it had its day?
Thanks for your help
Peter